In their simplest form, such respirators are comprised of a face piece formed from stabilized textile fibers, and which has been appropriately molded or otherwise formed for it to extend over the nose and mouth of a user, and, which is held in that position by elastic straps attached to the face piece and which are passed over the user's head to resiliently hold the respirator in situ over the user's nose and mouth.
Such dust masks are entirely temporary and fully disposable, in that once the filtering capability of the filter has been exhausted due to clogging of the filter, the user has no option other than to discard the dust mask and replace it with another, unused mask.
Refinements of such dust masks are disclosed in U.S. Pat. No. 4,319,567, Maggidson, U.S. Pat. No. 4,384,577, Huber, et al., and in U.S. Pat. No. 4,454,881, Huber, et al., each of which teaches stabilization and reinforcement of the filter in order to prevent collapse of the face piece under the partial vacuum produced by the user during inhalation, or, accidentally by the application of manual force of mishandling of the respirator.
While these constructions had an elegance absent from more mundane constructions, they still suffer from the same disadvantage that the entire respirator must be discarded after a relatively short period of use.
Further, in addition to impeding inhalation by the user as particulate matter builds up in the filter, such masks also progressively impede exhalation by the user through the clogged filter material, unless some form of exhalation valve is provided, as taught in U.S. Pat. No. 4,454,881, Huber et al. In the absence of an exhalation valve, a further physical stress is imposed on the user, particularly in the event that the user is involved in manually strenous work. Further, such respirators or dust masks have a relatively large internal volume that becomes filled with exhaled air as the user exhales, and which is then reinhaled when the user next inhales. Heat from the inhaled air raises temperature of the thick (extra thick in case of metal fume) filter fibers from exhalation. This heat is then transferred to the incoming air upon inhalation. A raise in temperature as low as 2.degree.-3.degree. causes great psychological and physical discomfort to the user.
An alternative approach to such respirators is disclosed in U.S. Pat. No. Des., 270,957, Maryyanek, which provides a face piece of soft rubber-like material, to which is attached a replaceable air filter in the form of a cartridge that snaps onto an inlet of the face piece controlled by a conventional inlet valve. The face piece is separately provided with dual conventional exhalation valves attached to the face piece at positions spaced from the inlet valve, and which are actuated by the rise in pressure in the face piece upon exhalation by the user.
In this construction, air inhaled through the filter does not have to be subsequently exhaled through the filter, but instead, is by-passed through the exhalation valves.
While this construction is successful in retaining disposability and replacement of the filter cartridge, it carries with it the disadvantage that the entire filter cartridge must be disposed of after use, with the attendant expense thereof, and also carries with it the disadvantage of the respirator being of considerable bulk and weight, and thus of perceptible inertia to movement of the user's head. Also, in this construction, the face piece and its associated exhalation valves present a relatively large internal volume within the face piece in which exhaled air is trapped, and, subsequently is re-inhaled by the user.
Replacement filter respirators of this latter type are referred to as semi-disposable respirators, in that the cartridge incorporating the filter material can be removed from the face piece for disposal and replacement by an unused cartridge, the face piece itself and its associated inhalation and exhalation valves being a permanently retained portion of the respirator.